An active pixel sensor typically takes the form shown in FIG. 1. A photosensor array 101 is disposed on a single chip substrate 100 with a number of columns 102 and rows 104. Each pixel has a photoreceptor 122, a follower transistor 124, and a selection transistor 126.
The pixels from the photosensor are coupled to one or more analog to digital converters 110 which convert the analog information 106 from the sensor 100 into digital output information 112. The analog to digital converters 110 are typically on the same substrate 100 with the image sensor 100. In a particularly preferred architecture, as shown, one analog to digital converter is associated with each column of the array. This system operates in column-parallel mode. At each clock cycle, an entire row of information is simultaneously output from the bank of analog to digital converters. The accuracy of the output image, which is collectively obtained from the output of all the analog to digital converters, is therefore dependent on the accuracy of the analog to digital converters. These devices, however, are limited in size. They must fit on the substrate. They also need to be relatively fast to maintain the processing speed.
The present invention teaches a system of introducing statistical processing into the A/D converters in order to improve the overall image quality. This is done according to the present invention by using A/D converters that are configured to oversample the input signal, find a centroid of the oversampled signal, and use the oversampling to enhance the accuracy.
The ADCs can operate with fewer bits than required for the total output, since oversampling is carried out.